1. Field of the Invention
The present invention relates to a display device using ferroelectric liquid crystal. More specifically, the present invention relates to a display device using a complex made of ferroelectric liquid crystal and a polymer material.
2. Description of the Related Art
In general, ferroelectric liquid crystal display devices utilize a liquid crystal phase such as a chiral smectic C phase. In the chiral smectic C phase, liquid crystal molecules form a layered structure so as to be aligned at an angle against a normal of the layered structure. In a bulk state, the liquid crystal molecules are arranged having a spiral structure; however, in a liquid crystal cell with a thickness smaller than a spiral pitch of the molecules, they are undone and bistably arranged as shown in FIG. 5. The ferroelectric liquid crystal has spontaneous polarization (Ps) in the direction perpendicular to the normal. Therefore, upon being applied with a voltage, they are rearranged in such a manner that their spontaneous polarization is directed along the applied electric field. When a pair of polarizing plates (i.e., a polarizer and an analyzer) are attached to such a ferroelectric liquid crystal display panel, displays in a transparent state or in an opaque state can be obtained (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 36, 899 (1980)).
The ferroelectric liquid crystal molecules are rearranged as described above by the interaction between the applied electric field and the spontaneous polarization of the molecules, which enables a fast response of the liquid crystal molecules on the order of microseconds. The ferroelectric liquid crystal molecules have the property of keeping a state under the application of a voltage even after power-down. This is a so-called memory property. A display per scanning line is performed at a high speed by utilizing the characteristics of the ferroelectric liquid crystal, such as a fast response and a memory property. Thus, simple matrix display devices with a large capacity can be produced.
FIG. 6 shows a basic configuration of a ferroelectric liquid crystal display device 11. The display device 11 has substrates 9 and 10 attached to each other via a sealant 6 with a liquid crystal material 7 interposed therebetween so as to have a cell thickness of about 1.5 .mu.m. The substrate 9 has an electrode film 2a made of indium tin oxide (ITO), an insulating film 3a, an alignment film 4a, and a polarizing plate 12a. The substrate 10 has an electrode film 2b made of ITO, an insulating film 3b, an alignment film 4b, and a polarizing plate 12b. The alignment films 4a and 4b are made of a polymer film such as polyimide and their surfaces are subjected to rubbing. The electrode films 2a and 2b are connected to driving circuits.
The display device 11 has the same configuration as that of conventional simple matrix liquid crystal display devices, except that the cell thickness is as thin as 1.5 .mu.m, and ferroelectric liquid crystal is used.
The ferroelectric liquid crystal display devices are weak with respect to shocks, a pressure, etc., so that they have a critical problem of instability (N. Wakita et al., Abstr. 4th International Conference on Ferroelectric Liquid Crystals, 367 (1993)). In general, liquid crystal molecules in a liquid crystal cell are likely to flow upon the application of shocks, a pressure, etc. The flow of the liquid crystal molecules disturbs the initial arrangement of the liquid crystal molecules. The weakness of the ferroelectric liquid crystal display devices with respect to shocks, a pressure, etc. is attributable to the fact that once disturbed liquid crystal molecules do not voluntarily return to their original arrangement. Thus, in order to enhance shock stability of the ferroelectric liquid crystal display devices, it is required to prevent the liquid crystal molecules from flowing in the liquid crystal cell upon the application of shocks, a pressure, etc.
The following methods have been proposed for preventing the liquid crystal molecules from flowing upon the application of shocks, a pressure, etc.
One method uses a polymer dispersed ferroelectric liquid crystal material (H. Molsen, R. Bardon, H. S. Kitzerow, The 13th international display research conference, Proc of Euro Display '93, Strasburg, 1993, p. 113). However, display devices which satisfy requirements for practical use thereof, such as a response speed, a contrast, and an available working temperature range have not been produced using this method.
Another method has been proposed in which polymer liquid crystal is added to ferroelectric liquid crystal (G. Lester, H. Coles, A. Murayama, Ferroelectrics, 148, 389 (1993)). This method also has a drawback in that the added polymer liquid crystal decreases the orientation property of ferroelectric liquid crystal molecules and makes a response speed thereof lower.
Recently, a method has been proposed, in which a mixture of ferroelectric liquid crystal and a light-curable prepolymer is irradiated with UV-rays, thereby phase-separating the mixture into ferroelectric liquid crystal and a polymer formed by photopolymerization (Fujikake et al., Preprints No. 3, 1120 (1994) of The 41st Applied Physics related association lecture meeting). According to this method, the minuteness of the formed polymer does not allow light to be scattered. This literature neither discloses nor suggests the enhancement of shock stability. In addition, according to this method, the mixture of ferroelectric liquid crystal and the light-curable prepolymer is heated so as to exhibit a nematic phase (i.e., heated to 75.degree. C.) and is phase-separated into a polymer and ferroelectric liquid crystal while the liquid crystal molecules are aligned with a polyimide film. In this manner, the liquid crystal molecules are fixed by the polymer without exhibiting a smectic C phase which they are supposed to exhibit. As a result, there arises a problem that the liquid crystal molecules are not sufficiently oriented.